The human body presents particular challenges for wireless signaling between body-mountable devices (that is, “wearable” devices) employing radio frequency (RF) signaling. The human body has a high permittivity, which introduces a detuning effect that changes the operational frequency of an antenna in proximity to the body. Moreover, the human body also has a high conductivity, which introduces a high dielectric loss and, as a consequence, reduces the antenna's radiating efficiency. As such, the high permittivity and high conductivity of the human body makes it difficult for RF signals at microwave frequencies to penetrate the human body, and thus leading to a shadowing effect that limits cross-body communications. Moreover, wireless wearable devices often may be worn and used while the user is in a large room or outdoors, and thus the wearable devices cannot reliably depend on multipath reflection for cross-body communications.
In view of these complexities, wearable device designers frequently rely on surface waves (also known as creeping waves) to establish cross-body communications between wireless wearable devices. One conventional antenna that is effective at generating such surface waves is the monopole antenna. However, as a monopole antenna relies on a radiating structure that projects orthogonally from a ground plane for a considerable distance, monopole antennas typically have a form factor that is impracticable for use in portable devices intended to be worn by the user. Other conventional antenna designs that have been attempted for cross-body signaling include patch antennas, slot antennas, inverted-F antennas (IFAs), and dipole antennas. However, the signaling effectiveness of each of these antenna designs is orientation dependent relative to the shortest path to the other antenna along the body surface, and it typically is not reasonable to expect the antennas on two different wearable devices worn by a user to maintain a constant orientation for cross-body communications.